How many photons does rhodopsin need to trigger a nerve impulse?

[rw11]

I am wondering how many photons must be captured by a rhodopsin molecule to energize it, and also to trigger a nerve impulse to the brain or to deeper in the eyes neural system?

 

[Pythagorean]

http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html

 

[Ygggdrasil]

It takes only one photon to isomerize the retinal in rhodopsin, and activate the rhodopsin molecule. Individual rod cells are capable of responding to an individual photon (http://journals.aps.org/rmp/abstract/10.1103/RevModPhys.70.1027), but as Pythagorean's link points out, some of the downstream neurons responsible for processing visual simuli may not be triggered by the signal from a single rod cell.